How to add search engine using Firefox's address bar?

If I go to a website offering dictionary features, e.g. dictionary.com, I can easily add this search engine to Firefox using the search bar.

Say that I'm using Firefox without the search bar. How can I add a search engine using the address bar?

There's no option in the address bar for it as far as I can see; to add this search engine I changed preferences, added the search engine, then removed the search bar again.

Solution 1:

See the Mozilla article Search the web from the address bar :

When you type in the address bar and press Enter Return, Firefox takes you to search results using the search provider selected in the search bar.

You may also use smart keyword as described in How to search IMDB, Wikipedia and more from the address bar.

Solution 2:

There is now an option on the page action menu to add a search engine. This is available from Firefox 61 onwards.

From the release notes of Firefox 61.0:

Convenient access to more search engines: You can now add search engines to the address bar “Search with” tool from the page action menu when on a webpage that provides an OpenSearch plugin